Method for fractionating air by liquefaction and rectification



y 2, 1955 J. WUCHERER ET AL 2,712,738

METHOD FOR FRACTIONATING AIR BY LIQUEFACTION AND RECTIFICATION Filed Jan. 2, 1953 2 Sheets-Sheet l nvvav TORS. c/OHA/VNES M/UCHER'EIE ATTORNEY.

y 2, 1955 J. WUCHERER ET AL 2,712,738

METHOD FOR FRACTIONATING AIR BY LIQUEFACTION AND RECTIFICATION Filed Jan. 2, 1953 2 Sheets-Sheet 2 Rue OL FBECKEE.

A7701? EX Uni NIETHOD FOR FRACTIONATING AIR BY LIQUEFACTION The present invention relates to an improved method and means for fractionating air by liquefaction and rectification.

The oxygen obtained from atmospheric air in conventional systems is usually without pressure. There are many cases, however, in which the oxygen is required at an increased pressure and must be compressed after it has been separated. Compression cannot be effected in conventional, lubricated compressors, requiring special measures which make it desirable that the oxygen be obtained already at an increased pressure.

A method has been proposed for obtaining oxygen by liquefaction and rectification of air. in this method the rectifying column is operated at normal pressure, the separated oxygen being compressed in liquid state and thereupon evaporated and heated to the ambient temperature by heat exchange with air to be fractionated. Thereby, the oxygen can be evaporated under pressure and its cold can be recovered by heat exchange with air Whose pressure is so high that it is liquefied at a temperature which is higher than the evaporation temperature of the oxygen. This method involves undesired losses. In order to avoid loss of cold, the difference between the temperature of the air and that of the oxygen must be small at the warm end of the heat exchanger. The air must be warmer than the oxygen at each point of the heat exchanger to effect heat exchange. Except at the Warm end, the temperature difference is smallest at the point of the heat exchanger where the oxygen begins to evaporate. Considering the smallest permissible temperature diflerence at the warm end of the heat exchanger and at the point where evaporation of the oxygen begins and considering the veloc given quantity of oxygen. If the amounts of air at a given pressure of the oxygen, are plotted in relation to increasing air pressures, the curve will have a flat minimum; the amount of air condensed during the heat exchange with the oxygen is always greater than the amount of oxygen; at the aforementioned minimum, the amount of air is in most cases to 30% greater than the amount of oxygen.

Since the total amount of the substances leaving the plant is equal to the amount of air which enters the plant the amount of air which exchanges heat is always smaller than the amount of nitrogen leaving the plant, increasing loss of cold. The conditions are more favorable in regenerators usually employed for the heat exchange between air the air being about equal to that in the prefractionating column, i. e. 5 to 6 atmospheres absolute. These heat exchangers are subject to similar limitations with respect to t. e relative amounts of the heat exchanging gases as the oxygen heat exchanger; in these cases, however, the amount of air may be somewhat up to 2.5%smaller than the amount of nitrogen; if the diiference is greater, the heat exchange is impaired, causing additional cold loss. If parts of oxygen are obtained by the decomposition rates Patent Patented July E2, 1955 of 100 parts of air, 20 1.25=25 parts of air must exchange heat with the oxygen. This leaves only parts of air for exchanging heat with parts of nitrogen, whereas 80 O.975=78 parts of air would he required. Economic operation is therefore not possible.

Economic operation can be effected only by increasing the pressure of a part of the total amount of air, whereby the specific heat is increased and heat can be exchanged with a greater amount of nitrogen and the aforementioned gap can be closed.

This solution of the problem, however, requires that a relatively large portion of the total airabout 35 to 55%must be compressed, which requires additional energy. i t is not advisable to use regenerators or similar self-cleaning heat exchangers for highly compressed air, because the air remaining and lost in these devices at every reversal of flow is too great. If counter flow heat exchangers are used, the relatively be dried and the carbon dioxide from, involving additional cost.

It has been proposed to increase the pressure of the total amount of air only to that at which it is fractionated and to employ a nitrogen cycle in which the pressure of the nitrogen is increased and the latter exchanges heat with the oxygen evaporating at increased pressure. In this way drying and separation of CO2 can be avoided, if the low pressure air heat exchangers (regenerators) are so constructed that amount of nitrogen does not contact the heat exchange surfaces which are coated with water ice and solid carbon dioxide, the diverted nitrogen being conducted to the compressor of the cycle. However, the aforementioned difhculties of the heat exchange for air are greater for nitrogen so that an additional heat exchange, cycle-nitrogen and expanded nitrogen, must be provided. The power requirements or" the conventional process employing a nitrogen cycle are still greater, than those of the process without a nitrogen cycle.

it is an object of the present invention to provide an improved method for ractionating air by liquefaction and rectification which method avoids the difliculties involved in the afore-described conventional methods. The new method is applicable to systems With and Without nitrogen cycle; the latter application will now be described.

It has been explained supra, that there is an optimal pressure for the heat exc ange between air and oxygen, at which pressure a minimum of excess air is required. This pressure is about 28 atmospheres absolute at a pressure of the oxygen of 12 atmospheres absolute and it is about 80 atmospheres absolute at a pressure of the oxygen of 25 atmospheres absolute.

of highly comup for the deficiency of air which would otherwise exist. sure of the oxygen is 25 atmospheres absolute, 53% of the air would have to be compressed to S atmospheres absolute, if no additional highly compressed air is used. If a portion of the air is highly compressed for the heat exchange with oxygen of 25 atmospheres absolute, only 24% of the air need be compressed to 80 atmospheres absolute and 3.5% would have to be compressed to 200 atmospheres absolute. The energy required in the second case is 23 less than in the first case.

Considerable savings can also be obtained, if the ideas underlying the present invention are applied to the closed cycle system, in which a gas is circulated whose thermodynamic characteristics, particularly vapor pressure and heat ot'evaporation, are similar to those of oxygen and which gas can be compressed in compressors lubricated by oil. Such a gas is argonits boiling point is at a pressure of 760 mm. at 87.5 Kelvin) and its heat of evaporation is 1500 xg-caL/mole (oxygen 1595 kg.-eal./ mole). its only disadvantage seems to be its much lower specific heat of 5.00 kg.-cal./C. mole (oxygen 7.01 kg.-cal./ C. mole). Argon, however, must be compressed substantially less than nitrogen. If the pressure of the oxygen is 25 atm. absolute, the pressure of argon need be only 33 atm. absolute whereas the pressure of nitrogen must be 100 atm. absolute. About the same excess amount of compressed argon is required for heat exchange with oxygen as excess air would be needed, if air were used for the heat exchange; this is primarily due to the low specific heat of the argon. Howor this low specific heat, there is no deis exchanged between expanded argon can absorb the heat of ever, because ficiency, if heat and air, because? parts argon only parts air at 5.5 atm. absolute. The disadvantage isthat one must employ a closed cycle, which, in contradistinction to nitrogen, does not take part in the rectification process, requiring additional heat exchange surfaces for the evaporation of the liquefied argon. Argon, however, can be obtained without diiiiculty'as a by-product when fractionating the air so that losses due to leakage in the closed cycle can be continuously replaced.

A better understanding of the invention will be afforded by the following detailed description considered in conjunction with the accompanying drawings, in which Fig. 1 is a diagrammatic illustration of a system according to the invention, all parts unessential to the invention being omitted;

Fig. 2 is a diagrammatic illustration of a modified system according to tl'ieinvention, in which parts unessential for the process according to the invention are also omitted. Referring more particularly to Fig. 1 of the drawing, the total amount of air is compressed in a turbocompressor i to 5.5 atm. absolute and the major part of the compressed air is conducted into one of two regenerators 2 and 2. Another part of the air passes through a device '3 for drying the air and separating carbon dioxide therefrom before it is compressed in an additional compressor 4 whichcompresses a portion of the air to 80 atm. absolute which portion is conducted'to a heat exchanger 5. Another portion of the air, which has passed through the device 3, is compressed to 200 atm. absolute and conducted into a heat exchanger 6. All three air streams are reunited after they have beencooled and after the pressure of the streams of air which has been compressed to 80 and 200 atm. absolute has been reduced in a valve '-7 and *5 respectively. The united air is conducted into a rectifier 9 which is only diagrammatically illustrated. The nitrogen, which is withdrawn from the top of the rectifier, conducted to the regenerator 2, a small portion of the nitrogen being conducted into the heat exchanger 6; The oxygen is in liquid state, compressed in pump 10 to the desired pressure,'and evaporated and heated to the ambient temperature in the heat exchanger 5.

'In the modification shown in Pig. 2, the air to be fraclf the pres-' Kelvin (oxygen 90.2"

withdrawn from the rectifier tionated is compressed in turbocotnpressor 11 and conducted into one of the regenerators 12 or 12' and therefrom into the rectifying apparatus 14 after it has been cooled and conducted through an argon evaporator 13, which will be described later. The nitrogen, separated in the rectifying apparatus 14, is conducted to the other of the regenerators 12 and 12. in each regenerator a pipe coil is provided through which argon of a pressure of about 2 atm. absolute and coming from the evaporator 13 is conducted. This avoids passage of argon through channels which have previously received water and separated carbon dioxide. The argon, after it has been heated to ambient temperature in the regenerators, is'compressed in a compressor 15 to 33 atm. absolute and thereupon conducted into a heat exchanger 16 in which it is cooled and liquefied. It is then expanded in valve lfand returned to the evaporator 13 where it is evaporated and where it liquefies a part of the air coming from one of the regenerators. The oxygen separated in the device 14 is compressed in pump 18 and evaporated and heated by heat exchange with compressed argon in the heat exchanger 6.

For replenishing the circuit and replacing argon losses,-

a small argon separating column 1% is provided whose product passes through the heat exchanger 16 and is compressed in a small auxiliary compressor 20 and passed into an accumulator 21. From the latter so much argon is passed into the argon circuit through a reducing valve 22 as is needed to replace losses. The accumulator 21 may be omitted, if desired.

Since the amount of argon circulating in the closed circuit is smaller than the amount of nitrogen in a nitrogen circuit at approximately the same compression ratio, the power requirements are considerably less than those of a nitrogen circuit and are not much greater than when producing oxygen without pressure and subsequent compression of the oxygen. The amount of argon circulating in the circuit is in the order of 24% of the amount of air to be decomposed, whereas the amount of nitrogen circulating in the nitrogen cycle is 33%.

What is claimed is:

1. A process for fractionating air by liquefaction and rectification in which the separated oxygen is compressed while in liquid state and subsequently evaporated and heated to ambienttemperature by heat exchange with the air to be fractionated, said process comprising the steps of dividing the air to be fractionated into three streams, compressing the air of each stream to a different pressure, cooling the streams of air having the highest and the lowest pressure by heat exchange with the separated nitrogen, and cooling the stream of air having a pressure between that of the streams of highest and lowest pressure by heat exchange with the separated oxygen.

2. A process as defined in claim 1, the pressure of the stream of airof lowest pressure after-ding liquefaction or" the separated nitrogen by heat exchange with separated oxygen evaporating during the rectification, and the pressure of the air stream exchanging heat with the separated oxygen and the pressure of the air in the stream of highest pressure being such as to afford a minimum of air flowing in the two last mentioned streams.

References Cited in the file of this patent UNITED STATES PATENTS 2,433,508 Dennis Dec. 30, 1947 2,480,094 Anderson Aug. 23, 194.9 2,496,380 Crawford Feb. 7, 1950 2,501,999 Pause-k Mar. 28, 1950 2,502,282 Schlitt Mar. 28, 1959 2,640,332 Keyes June 2, 1953 2,667,043 Collins Ian. 26, i954 FOREIGN PATENTS 469,939 Great Britain Aug. 3, 1937 

1. A PROCESS FOR FRACTIONATING AIR BY LIQUEFACTION AND RECTIFICATION IN WHICH THE SEPARATED OXYGEN IS COMPRESSED WHILE IN LIQUID STATE AND SUBSEQUENTLY EVAPORATED AND HEATED TO AMBIENT TEMPERATURE BY HEAT EXCHANGE WITH THE AIR TO BE FRACTIONATED, SAID PROCESS COMPRISING THE STEPS OF DIVIDING THE AIR TO BE FRACTIONATED INTO THREE STREAMS, COMPRESSING THE AIR OF EACH STREAM TO A DIFFERENT PRESSURE, COOLING THE STREAMS OF AIR HAVING THE HIGHEST AND THE LOWEST PRESSURE BY HEAT EXCHANGE WITH THE SEPARATED NITROGEN, AND COOLING THE STEAM OF AIR HAVING A PRESSURE BETWEEN THAT OF THE STREAMS OF HIGHEST AND LOWEST PRESSURE BY HEAT EXCHANGE WITH THE SEPARATED OXYGEN. 